Rectangular cathode ray tube having post acceleration helix curved to compensate for orthogonality errors



E. E. HIMMELBAUER ET AL 3,5 ,095 RECTANGULAR CATHODE RAY TUBE HAVING POST ACCELERATION Aug. 11, 1970 HELIX CURVED TO COMPENSATE FOR ORTHOGONALITY ERRORS Filed Nov 14. 1967 AGEN Int. Cl. H01j 29/46, 29/80 US. Cl. 313-83 1 Claim ABSTRACT OF THE DISCLOSURE A cathode ray tube having a conductive helical coating in the conical portion of the envelope for producing a post acceleration field. In order to correct orthogonality errods due to asymmetrical deflections of the electron beam, the helix is provided with curvatures extending in the directions of the electron gun and the screen.

This invention relates to a cathode ray tube having an electron gun for producing an electron beam, a screen for receiving the electron beam, and deflection means for deflecting the electron beam in two directions substantially orthogonal to one another, the vacuum envelope of said tube having a conical portion located between the deflection means and the screen and the inner side of which is covered with a conductive helical coating for producing a post-acceleration field.

It is common practice to provide a so-called postacceleration field between the deflection means and the screen in cathode ray tubes in which the electron beam, after being deflected, strikes a screen on which the beam can be visually observed, for example, in oscillograph tubes and display tubes. Thus the electrons in the region between the deflection means and the screen acquire a high velocity, resulting in great luminosity of the luminous spot on the screen, whereas the velocity of the electrons in the region in which they are deflected may be comparatively low so that a high deflection sensitivity can also be obtained. The post-acceleration field is usually produced by means of electrodes provided on the inner side of the envelope of the tube.

An electrode for producing a post-acceleration field in a cathode ray tube comprises a conductive helical coating on the inner side of a conical portion of the tube.

In the manufacture of the envelope of a cathode ray tube, systematic deviations in the circular shape of the cross-sections of the portion of the envelope carrying the post-acceleration helix will in many cases arise, for example, due to deformation resulting from the opening of a mold in which the envelope cannot be turned. These systematic deviations will be diflicult to avoid, especially in the manufacture of tubes having a non-circular screen, such as, for example, tubes having a rectangular screen. Such deviations in the circular shape of the cross-sections of the portion of the envelope carrying the post-acceleration helix will introduce deviations in the symmetry of the post-acceleration field produced by the post-acceleration helix. Consequently, unwanted deviations will arise in the pattern reproduced on the screen. It is possible that the deflection sensitivities are influenced very diflerently in different directions of deflection. An important deviation in the orthogonality of the directions of deflection may also be introduced. A deviation in the orthogonality of United States Patent 0 "ice the direction of deflection is to be understood to mean hereinafter a deviation in the orthogonality of the directions of deflection such as are reproduced on the screen. Deviations in the orthogonality of the directions of deflection are highly undesirable, especially in oscillograph tubes, in which the distances between the luminescent points on the screen and two relatively perpendicular axes are ,used as the result of measurement and hence have to correspond to the actual deflection.

An object of the invention is to provide a cathode ray tube having a post-acceleration helix having a shape such as'to' compensate for deviations in the pattern reproduced on v the screen, which deviations result from the noncircular shape of the cross-sections of the portions of the envelope on which the post-acceleration helix is provided.

According to the invention, the conductive helical coating has several curvatures towards the electron gun and, towards the screen.

Thus, in a cathode ray tube according to the invention, hills and dales are present in the helical coating on the inner surface of the envelope. The electrical field inside thefhelix, which is produced by means of the helix if a potential dilference is applied to its ends, may be recorded as the resultant of three components, namely:

('1) The field of a helix on an imaginary ideal cone the cross-sections of which are exactly circular and each have, for example, a radius which is equal to the mean radius of the corresponding cross-section of the cone of the tube;

.(2) The field resulting from the deviations in the circular shape of the cross-sections of the cone; in other words, the field resulting from the radial deviations of the helix;

(3) The field resulting from the said hills and dales in the helix; in other words, the field resulting from the axial deviations of the helix.

Deviations in the pattern reproduced on the screen, which deviations are caused by the radial deviations of the helix, may be neutralized by means of suitably chosen axial deviations of the helix.

This is especially the case if the axial deviations of the helix are dependent upon the radial deviations of the helix so that the said third component of the field sub stantially neutralizes the deviations in the symmetry of the field within the helix relative to the axis of the tube, caused by the said second component of the field. In this case any deviations in the shape of the pattern reproduced on the screen, caused by the radial deviations of the helix are substantially neutralized.

However, the invention is not limited to this case in which there are axial deviations of the helix to bring about the symmetry of the field. It also relates to a cathode ray tube in which the field resulting from the axial deviations of the helix compensates for deviations in the pattern reproduced on the screen, caused by the radial deviations of the helix, Without the field resulting from the axial deviations eliminating the deviations in symmetry. In this tube, the axial deviations are chosen as a function of the kind of the deviations in the pattern on the screen which it is desired to suppress. The deviations in orthogonality will generally be seen as the most important deviation in the pattern resulting from the radial deviations of the helix. Consequently, the invention particularly relates to a cathode ray tube in which axial deviations of the helix brought about in a simple manner, compensate for the deviation in orthogonality resulting from the radial deviations of the helix.

The invention especially relates to a cathode ray tube in which the conductive helical coating, during each revolution about the axis of the tube, is curved alternately, twice in each direction towards the electron gun and towards the screen, the resulting relative extremes of the curvature of the helix being located in the planes which pass through the axis of the tube and are at angles of substantially 45 to the directions of deflection.

In this tube the hills and dales present in the helical spiral produce a quadripolar field having planes of symmetry which pass through the axis of the tube and are at angles of substantially 45 to the directions of deflection. Such a quadripolar field causes a shift in the directions of deflection such as appear on the screen. The two directions shift as a result of such a quadripolar field in opposite senses so that the angle between the two directions is varied. By suitable choice of the positions of the hills and the dales the quadripolar field compensates for the deviations in the orthogonality of the direction of deflection, introduced by deviations in the circular shape of the cross-sections of the conical portion of the tube.

The invention especially relates to a cathode ray tube of the type above-mentioned in which the hills and the dales in the path of the helix have a height such that, if a potential difference exists between the ends of the helical coating, this coating does not introduce deviations in orthogonality. With correct positioning of the hills and dales their height can always be chosen in this way. The magnitude of the shift resulting from the quadripolar field depends upon the post-acceleration voltage and upon the shape and height of the hills and the dales. The magnitude of the deviation in orthogonality resulting from the deviations in the circular shape of the cross-sections of the envelope has, apart from the sign, substantially the same dependence upon the post-acceleration voltage as the extent of the shift resulting from the hills and dales. Thus, the extent of correction is substantially independent of the value of the post-acceleration voltage. With the shape determined, the height is determinative of the extent of shifting. It is possible to determine by experiment the height at which the deviation in orthogonality resulting from the deviation in the circular shape of the cross-sections of the conical portion of the tube is exactly fully neutralized, in other words, the height at which the helix (for a given shape of the hills and dales) just does not introduce a deviation in orthogonality.

The invention also relates especially to a cathode ray tube of the type above-mentioned which has an envelope the conical portion of which adjoins, at one end, a cylindrical portion which houses the electron gun and at the other a rectangular window with an upright edge on the inner side of which the screen is present.

In this tube the cross-section of the conical portion of the envelope will exhibit considerable systematic deviations in the circular shape of the cone. When using conventional helices, this could readily give rise to systematic deviations in orthogonality of approximately several degrees. These systematic deviations in orthogonality may be eliminated in a simple manner by means of the described shape of the helix for which the height of the hills and dales have been deter-mined empirically.

The invention will be described with reference to the accompanying drawing, in which:

FIG. 1 is an elevational view of a cathode ray tube having a right-angled window;

FIG. 2 shows several component parts of the cathode ray tube of FIG. 1.

FIG. 1 shows the glass envelope of the cathode ray tube having a conical portion 1, provided with a conductive coating -12, adjoining at one end a cylindrical portion 2 which houses the electron gun (not shown in FIG. 1) and at the other end a rectangular window 3 having an upright edge 4. The broken lines shown on the window 3 indicate the directions of deflection such as appear on the screen which is present on the inner side of the window. Conductive coatings 13 and 14 are provided before and behind the helix.

FIG. 2 shows a portion of the screen 5. It also shows the electron gun 6 for producing an electron beam which, if not deflected, passes along the axis of the tube and strikes the screen 5 at 0. Two deflection means and part of the conical portion 1 are shown between the electron gun 6 and the screen 5. The first deflection means comprises deflection plates 8 and 9 and serves to deflect the electrons in the direction A-B. The second deflection means comprises deflection plates 10 and 11 and serves to deflect the electrons in the direction CD, which is orthogonal to the direction A-B. The conductive coating 12 on the conical portion of the envelope has the shape of a helix on which a sinusoidal veriation is superimposed. The maxima of the sine wave lie at the areas at which the helix intersects the plane passing through the axis of the tube and the line EF which is at angles of 45 to the directions of deflection AB and CD. The minima of the sine wave lie at the areas at which the helix intersects the plane passing through the axis of the tube and the line GH which is likewise at angles of 45 to the directions of deflection of deflection AB and CD. The electrical supply leads to the electrodes, as well as their fastening means are omitted in the figures.

The numerical values of various magnitudes in this tube are, for example, the following: The conical portion 1 of the envelope, insofar covered by the helix, has a length of 120 mm. and has at one end a cross-section of which the largest internal dimension is 49 mm. and the smallest internal dimension is 46 mm. and, at its other end, a cross-section of which the largest internal dimension is mm. and the smallest internal dimension is 101 mm. The window 3 has dimensions of 95 mm. x mm. The helix has a Width of 1.3 mm. and a pitch of 1.9 mm. The post-acceleration voltage applied between the ends of the helix is thrice the voltage between the cathode and the beginning of the helix. The amplitude of the sinusoidal variation of the helix is 0.8 mm. Its magnitude has been determined empirically by finding out the amplitude at which the deviation in the orthogonality of the direction of deflection resulting from the deviations in the circular shape of the cross-sections of the conical portion 2 of the envelope is exactly eliminated. It was found that the deviation in the circular shape of the cross-sections of the conical portion of the envelope resulted in a systematic deviation in orthogonality of 3, which deviation is exactly eliminated by means of the said sinusoidal variation having an amplitude of 0.8 mm.

While the invention has been described with reference to a particular embodiment thereof, other modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claim.

What is claimed is:

1. A cathode ray tube comprising an envelope having a given axis and a cylindrical portion at one end housing an electron gun, a substantially elliptical conical portion having an end adjacent said cylindrical portion and another end adjacent a rectangular window having diagonals substantially parallel to the major and minor axes of the elliptical cross section of said conical portion, said window, further, having a screen luminescing in response to energization by said electron gun, a deflection system between said electron gun and screen for deflecting electrons from said gun in two substantially orthogonal directions, and helical conductor on the inner wall of said conical portion for producing a post deflection accelerating field, said conductor extending from said end adjacent said cylindrical portion toward said screen and curving alternately, twice in each direction, towards said gun and towards said screen, upon each revolution about said given axis, the relative extremes of the path of said conductor being located in planes passing through said axis at angles of substantially 45 to said orthogonal directions and being of a magnitude, with a predetermined potential difference between the ends of said conductor,

5 6 compensating for pattern deviations on said screen re- 3,243,645 3/1966 Parnes et a]. 313-86 X sulting from asymmetries in said conical portion. 3,270,234 8/1966 Schaffernicht et a1. 313-83 References Cited ROBERT SEGAL, Primary Examiner UNITED STATES PATENTS 5 Us CL XR. 2,899,600 8/1959 Bryan. 313-86 3,143,681 8/1964 Schlesinger 313-83 

